This invention relates generally to sol-gel processes for producing dry gel monoliths and, more particularly, to drying processes and apparatus for rapidly drying wet gel monoliths without inducing cracking.
High-purity glass and ceramic components typically are fabricated either by a melting of solid raw materials or by vapor deposition. Melting of solid raw materials is a generally effective technique, but difficulty is encountered in maintaining purity, due to the inherent presence of impurities in the raw materials and due to recontamination from processing containers at the high melting temperatures. In addition, energy costs due to high temperature processing can sometimes be excessive, and finishing costs to produce components of the desired final shapes also can be excessive. Vapor deposition likewise is generally effective, but very expensive due to a relatively low (e.g., 50%) material collection efficiency, a high investment cost in processing and pollution control equipment, and slow processing rates.
High-purity ceramic components are typically fabricated by processes such as solid extrusion and colloidal casting. Like high-purity glass fabrication processes, these processes also require high temperature processing and the articles fabricated are limited in composition, homogeneity and purity.
Research has recently been conducted into the use of sol-gel processes for fabricating high-purity monolithic articles of glass and ceramic. In such processes, a desired solution, i.e., sol, of glass- or ceramic-forming compounds, solvents, and catalysts is poured into a mold and allowed to react. Following hydrolysis and condensation reactions, the sol forms a porous matrix of solids, i.e., a gel. With additional time, the gel shrinks in size by expelling fluids from the pores. The wet gel is then dried in a controlled environment, to remove fluid from its pores, and it is then densified into a solid monolith.
Advantages of the sol-gel procedss include chemical purity and homogeneity, flexibility in the selection of compositions, processing at relatively low temperatures, and producing monolithic articles close to their final desired sol-gel process has generally proven to be extremely difficult to use in producing monoliths that are large and free of cracks. These cracks arise during the final drying step of the process, and are believed to result from stresses due to capillary forces in the gel pores. Efforts to eliminate the cracking problem present in sol-gel monoliths have been diverse. However, the problem of cracking has not previously been eliminated without sacrificing one or more of the benefits of the process, as listed above.
One technique for eliminating cracking during the final drying step of the glass or ceramic gel is to dry the gel above its critical temperature, with a suitable fluid in an autoclave. Above the critical temperature and pressure, there is no vapor/liquid interface in the pores and thus no capillary force exists. The fluids are removed from the pores while in this condition, and a dry gel is thereby obtained. Although this technique is effective, it can be dangerous and it requires relatively expensive equipment.
Another technique for eliminating cracking during the final drying step is to increase the pore size distribution by using various catalysts. However, this approach has not proven to be particularly successful for large monoliths, because no catalyst is believed to have been found to produce average pore sizes above about 100 Angstroms.
Yet another technique for eliminating cracking during the final drying step is to add colloidal silica particles to the sol, which increases the average pore size and correspondingly increases the solid matrix's strength. Although this technique is generally effective, the presence of colloidal silica particles sacrifices the gel's otherwise inherent homogeneity, thus restricting the range of compositions that can be utilized. In addition, devitrification spots can be created, if mixing of the colloidal silica particles is not perfect.
Yet another technique for eliminating cracking during the final drying step is to add drying control additives to the sol, to produce a more uniform pore size distribution and thereby strengthen the gel matrix. These drying step. Although generally effective in eliminating cracking, this technique generally produces monoliths having a large number of bubbles.
Yet another technique for eliminating cracking during the final drying step is to hydrothermally age the gel prior to drying, which increases the average pore size in the gel and correspondingly decreases the capillary stresses encountered during drying. Although this technique is generally effective, the aging step increases the time and the equipment cost for drying gels and thus increases the cost of the final product.
Still another technique for eliminating cracking during the final drying step is to heat the gel to a subcritical temperature in a chamber having several pinholes to allow the evaporating fluid to escape. Although generally effective, this technique can be very slow, requiring months to complete. The drying rate can be increased by increasing the area of the pinholes, but this frequently leads to cracking.
It should, therefore, be appreciated that there is a need for an improved drying process for producing large glass and ceramic monoliths that are substantially free of cracks, without sacrificing other benefits attendant to the sol-gel process, such as low relative expense, chemical purity and homogeneity, flexibility in the selection of compositions, and low temperature processing. The present invention fulfills this need.